High-Temperature and Drought Tolerance of Tropical Mountain Forests and Lowland Rainforest Trees in Southeast Asia

Universiti Putra Malaysia, Malaysia

Forest Department Sarawak, Malaysia

Nanyang Technological University, Singapore

Kochi University, Japan

FY 2016-2018 Grant-in-Aid for Scientific Research (KAKENHI)

TANAKA, Kenzo

High temperatures and drought are likely to occur more frequently in Southeast Asia due to climate change, which may have significant effects on the forest dynamics and biodiversity. However, only limited studies have been conducted in drought responses on tree species in the lowland rainforests and tropical mountain forests of Southeast Asia. In addition, there is little knowledge about the high-temperature tolerance of tropical trees, aside from a few reports on the temperature plasticity of photosynthesis in several species of Dipterocarpaceae in Southeast Asia. This lack of information on the high-temperature and drought tolerance of these trees will become a serious problem in the future in terms of the sustainable management, restoration, and regeneration of these forest ecosystems.

The aim of this research was to investigate the physiological and ecological responses of major tree species to high temperatures and drought by undertaking a number of experiments, such as artificial rainfall exclusion experiments and transplantation experiments in which plants were moved from low-temperature to high-temperature conditions. The results obtained will contribute to predicting the response of tropical forests in Southeast Asia to future climate change.

We conducted this study in lowland tropical rainforests in Malaysia and Singapore and tropical mountain forests in Malaysia. In the lowland tropical rainforests, we examined the high-temperature and drought tolerances of the seedlings of typical tree species by transplanting them from low-temperature and shaded conditions to high-temperature and light conditions and also evaluated the drought tolerance of mature trees through a rainfall exclusion experiment. In addition, we compared the moisture content of the wood between trees growing in a tropical rainforest and tropical seasonal forest and considered the relationship between this and drought stress. In the tropical mountain forests, we selected representative tree species and transplanted their seedlings to high-temperature lowland conditions.

For the lowland tropical rainforest species, there was a large difference in seedling mortality among seedlings of three Aquilaria species under high-temperature and drought conditions. We found that tolerance to environmental stress was related to the leaf water use and morphological traits of the plants, particularly the photosynthetic water use efficiency and leaf mass per area, which are related to leaf thickness. These findings suggest that the leaf morphology could be used as a simple index for evaluating stress tolerance among Aquilaria species. We also compared the changes in leaf water use among adult lowland tropical rainforest trees during a drought event by installing a 30-m-diameter umbrella around three adult Dryobalanops aromatica (Dipterocarpaceae) trees growing in the rainforest in Borneo to exclude rainfall water and thus induce soil drought. We found that this species was able to maintain photosynthesis and transpiration during the morning under dry conditions by adjusting the osmotic potential in their leaves, whereas these activities decreased by approximately half in the afternoon. However, the stomata were still not completely closed at this time and thus photosynthesis and transpiration did not cease. This strategy of continuing to consume water under drought conditions will represent a high risk to survival under a severe and prolonged drought event.
We also found that when seedlings of mountainous forest tree species in the tropics were transplanted from the cool environment in which they normally grow to a high-temperature lowland environment, they retained almost the same photosynthetic capacity but experienced a reduced growth rate due to an increase in respiration rate. We believe that this will be a disadvantage when the growth of these tree species is limited due to global warming as they will need to compete with lowland forest tree species. Together, these findings contribute to the prediction of future forest dynamics and the quantitative evaluation of the high-temperature and drought acclimation of tropical forest trees.

Information about the ability of different species to acclimate to high temperatures and drought will contribute to the better selection of appropriate tree species and the development of planting techniques under increased levels of stress. The physiological and growth parameters that were obtained under high-temperature and drought conditions can also be used as better parameters for use in vegetation models to understand the effects of climate change on forest ecosystems.

Ichie T, Inoue Y, Takahashi N, Kamiya K, & Kenzo T (2016) Ecological distribution of leaf stomata and trichomes among tree species in a Malaysian lowland tropical rain forest. Journal of Plant Research, 129, 625-635.

Kenzo T, Ichie T, Norichika Y, Kamiya K, Nanami S, Igarashi S, Sano M, Yoneda R & Lum S K (2016) Growth and survival of hybrid dipterocarp seedlings in a tropical rain forest fragment in Singapore. Plant Ecology & Diversity, 9, 447-457.

Kenzo T, Iida SI, Shimizu T, Tamai K, Kabeya N, Shimizu A, & Chann S (2016) Seasonal and height-related changes in leaf morphological and photosynthetic traits of two dipterocarp species in a dry deciduous forest in Cambodia. Plant Ecology & Diversity, 9, 505-520.

Tanaka-Oda A, Kenzo T, Inoue Y, Yano M, Koba K, & Ichie T (2016) Variation in leaf and soil δ15N in diverse tree species in a lowland dipterocarp rainforest, Malaysia. Trees, 30, 509-522.

Inoue Y, Ichie T, Kenzo T, Yoneyama A, Kumagai TO, & Nakashizuka T (2017) Effects of rainfall exclusion on leaf gas exchange traits and osmotic adjustment in mature canopy trees of Dryobalanops aromatica (Dipterocarpaceae) in a Malaysian tropical rain forest. Tree Physiology, 37, 1301-1311.

Kenzo T, Sano M, Yoneda R, & Chann S (2017) Comparison of wood density and water content between dry evergreen and dry deciduous forest trees in central Cambodia. JARQ, 51, 363-374.

Kenzo T, Kamiya K, Ngo KM, Faizu N, Lum SKY, Igarashi S, Norichika Y & Ichie, T (2019) Overlapping flowering periods among Shorea species and high growth performance of hybrid seedlings promote hybridization and introgression in a tropical rainforest of Singapore. Forest Ecology and Management, 435, 38-44.

Kenzo, T., Yoneda, R., Tanaka-Oda, A., & Azani, M. A. (2019). Growth performance and leaf ecophysiological traits in three Aquilaria species in Malaysia. New Forests, 50, 699–715. https://doi.org/10.1007/s11056-018-09693-7